feat: add backfill-specific parallelism control for streaming jobs

[shanicky]

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What's changed and what's your intention?

Introduce a new configuration and mechanism to specify separate parallelism settings for backfill operations in streaming jobs. This enables more efficient resource utilization by allowing different parallelism for backfill phases versus normal streaming.

• Add streaming_parallelism_for_backfill session config parameter
• Extend stream fragment graph and protobuf to carry backfill parallelism
• Modify stream fragmenter and DDL controller to apply backfill parallelism
• Implement global stream manager logic to restore normal parallelism after backfill
• Add integration tests verifying parallelism switching before and after backfill

dev=> set STREAMING_PARALLELISM = 3;
SET_VARIABLE
dev=> set STREAMING_PARALLELISM_FOR_BACKFILL = 2;
SET_VARIABLE
dev=> create table t(v int);
CREATE_TABLE
dev=> insert into t select * from generate_series(1, 10);
INSERT 0 10
dev=> set BACKFILL_RATE_LIMIT = 1;
SET_VARIABLE
dev=> set BACKGROUND_DDL = true;
SET_VARIABLE
dev=> create materialized view m as select * from t;
CREATE_MATERIALIZED_VIEW
dev=> show jobs; select name, parallelism from rw_fragment_parallelism where name = 'm';
 Id |                   Statement                   | Create Type |   Progress
----+-----------------------------------------------+-------------+---------------
  7 | CREATE MATERIALIZED VIEW m AS SELECT * FROM t | BACKGROUND  | 40.00% (4/10)
(1 row)

 name | parallelism
------+-------------
 m    |           2
(1 rows)

dev=> show jobs; select name, parallelism from rw_fragment_parallelism;
 Id | Statement | Create Type | Progress
----+-----------+-------------+----------
(0 rows)

 name | parallelism
------+-------------
 m    |           3
(1 rows)

Checklist

• I have written necessary rustdoc comments.

Documentation

• My PR needs documentation updates.

Release note